A subscriber-only article in the Economist says:

Zhou Xiaozheng, a sociologist at the People's University of China in Beijing, says the government has tended to treat protests by unpaid migrants with restraint, fearful that being tougher might only make things worse. But he argues that the practice of withholding wages from migrant labourers will persist as long as such people are treated as second-class citizens in urban China. �China has 10m slaves. The definition of a slave is someone who is given work and food but no wages. That's what these people are.�

That says it all, really.